[@Reanimator Bob]

Basically, Arthur can use any magic that a Witch, Warlock, Sorcerer or Wizard can use as long as it's at it's lowest level, without learning it, at will. For instance, if there's a wizard who could cause a blizzard of ice to cover all of New York, Arthur could freeze a large house hold, maybe a manor or rich guy's home at most. Or less, perhaps, maybe he could just send a chill wind at his enemies that make it very difficult and frigid for them to move. 

For more powerful magical abilities, things that take people hours, days, maybe weeks, months or even years to learn, Arthur actually learns them in an hour. He might still have to use the right objects and movements with hands and the use of spoken magic words to make a spell work, but he could use them effectively almost instantaneously. However, even then, he can use this magical power at it's weakest basic level at will. 

So Zeus'es thunderbolt, if it were written down in a Grimoire, might be more like a jolt of electricity strong enough to knock someone out instead of being capable of vaporizing giants and titans. As for his ability to just shape energy into magical abilities, he can conjure a magical shield made of unexplained energy to protect him, and fire a very strong projectile bludgeoning attack that could easily blast through a cutting edge military bunker in one hit. Like wise he could defend against extremely powerful spells with his forcefield, but the more powerful the magic he has to use, the more concentration it takes. 

Finally, even his concentration has limits, and he can fail. IF he has an enemy that he marks as prey, he mystically learns of their fears and weaknesses so he can track them down and destroy them. Finally, if he's suspicious that magical abilities can't help him, he still has the power to channel super human strength and physical power great enough to astonish anyone who is new to the supernatural world. He's not as strong as the Hulk, but he is stronger than Spiderman who can lift up to ten tons. . . He can physically empower himself to lift up to seventy tons as per physical strength, but he would lapse into a berserker rage where he would have to be restrained or incapacitated somehow. 

In gaining power from Cernunnos he can demonstrate up to ten tons without any loss of control. Up to twenty tons, he starts to become just slightly less in control of his mental faculties. Thirty tons, he is still only a little more foggy than usual, but he might accidentally land an unfortunate blow on a comrade. Forty tons, he could start to lose control as he tries to destroy his enemy, and could end up harming an ally severely when he's only trying to to fight. At fifty tons, he is in danger of lapsing into a berserker rage where he doesn't care if he starts to suffer bodily injury. At sixty tons, he's forty percent likely to succumb to a berserker rage and could very easily wind up doing something like throwing cars, tanks, and trucks, ending up with the unintended deaths of innocent bystanders. At seventy tons, he is sixty five percent likely to lose control of himself and needs to be put to sleep with a spell, some kind of technological tranquilizer, exotic techniques, etcetera, or if necessary, even death.